Antibacterial activity of gold nanoparticles/zinc oxide (AuNP/ZnO) hybrid nanostructures

Zinc oxide (ZnO) nanoparticles are widely used in biomedicine due to their notable antimicrobial properties. The most effective method to enhance its efficiency is by combining it with noble metals like gold nanoparticles (AuNP). This forms hybrid nanostructures that improve catalytic reactions by inhibiting electron–hole recombination and broadening the absorption spectrum. This study evaluates the antibacterial activity of the AuNP/ZnO hybrid nanostructure. The physicochemical properties of the as-prepared hybrid nanostructures were evaluated using various characterization techniques, including scanning electron microscopy (SEM), transmission electron microscopy with energy-dispersive spectroscopy (TEM-EDS), ultraviolet–visible (UV–Vis) spectroscopy, dynamic light scattering (DLS), and Fourier transform infrared spectroscopy (FTIR). SEM analysis revealed changes in the size and shape of the hexagonal ZnO rod matrix with varying concentrations of AuNPs incorporated into the hybrid. TEM-EDS confirmed the effective attachment of AuNPs, averaging 12.29 nm in size, to ZnO nanostructures, resulting in spotted ZnO nanorods, and this attachment is facilitated by the gradual chemisorption of functionalized nanoparticles. UV–Vis spectroscopy revealed a distinct blue shift in peaks from 365 nm for pristine ZnO to 354 nm in the hybrid nanostructure, indicating an energy band gap of 3.01 eV. DLS analysis revealed hydrodynamic sizes and agglomeration behavior, with AuNPs exhibiting a hydrodynamic size of 26 nm. An increase in size from ZnO to the hybrid nanostructures was observed, suggesting the formation of functional groups stabilized by weak van der Waals forces and hydrogen bonds. FTIR spectroscopy highlighted Zn–O stretching at 606 cm−1 and band shape and position alterations in the hybrid nanostructures. XRD analysis confirmed the successful surface incorporation of AuNPs onto ZnO without altering its hexagonal wurtzite crystal structure, indicating high crystallinity and phase purity. Disk diffusion tests demonstrated the antibacterial activity of AuNP/ZnO hybrids against both Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria, with average inhibition zones of 14.67 mm and 18.67 mm, respectively. E. coli exhibited greater resistance due to its unique cell wall structure, while S. aureus was more susceptible, allowing easier penetration of the hybrid nanostructures and increasing cell death. This highlights the significant potential of the hybrids in combating both bacteria and the potential development of antibiotic-resistant strains.